The present invention generally relates to spacecraft ephemeris determination. More specifically, the present invention relates to utilizing ranging information and ephemeris of other spacecraft to determine spacecraft ephemeris.
Orbit determination and control are important aspects of most satellite systems. The guidance and navigation function, also known as the Orbit Determination and Control Subsystem (hereinafter "ODCS"), measures and maintains the position of the spacecraft's center of mass. The position (and optionally the velocity) of a spacecraft as a function of time is commonly referred to as the spacecraft ephemeris. The ODCS determines the spacecraft's position in space using sensors. External references must be used to determine the absolute position of the spacecraft. The external references may include the Sun, the Earth's infrared horizon, magnetic fields, and the stars.
Next, the ODCS must control the orbital position of the spacecraft using actuators, such as gas jets or thrusters. The ODCS deals with spacecraft dynamics and has the multiple functions of acquisition, determination, maintenance, and maneuver control. Orbital control is needed whenever a satellite is trying to achieve a desired orbit. Orbital control is also needed to overcome orbit perturbations to achieve altitude maintenance in Low-Earth Orbit (hereinafter "LEO") or geosynchronous stationkeeping. Maintaining relative satellite positions, such as in constellation maintenance, also requires orbital control.
Navigation has two basic purposes. As just mentioned, navigation provides the information necessary to maintain and control an orbit, just as attitude determination provides the information necessary for attitude control. Thus, a requirement for orbit control will ordinarily result in a corresponding requirement for navigation. Navigation information may also be used in processing data from the payload.
Irrespective of orbit control, there is often a need to point an antenna or instrument in some direction to perform communication or observation tasks. For example, in satellite systems designed to track objects, knowledge of the positions of the satellites sensing the objects being tracked may be critical. Since the position of an object being tracked is ultimately derived from the position in space of the satellite(s) sensing the object, the accuracy of the tracking is directly dependent upon the accuracy with which the position of the satellite(s) is known.
In an ideal setting, ephemeris would always be as expected and there would be no need for complex and expensive determination and control subsystems. In reality, however, forces act on the satellite to move it away from the nominal orbit. There are short-period orbital variations (also known as "perturbations") that are periodic with a period less than or equal to the orbital period, and there are long-period perturbations, which are orbital variations with a period greater than the orbital period. There are also secular variations, which represent a linear orbital variation that increases over time. The primary forces that perturb a satellite orbit arise from third bodies such as the Sun and the Moon, the non-spherical mass distribution of the Earth, atmospheric drag, and solar radiation pressure.
In the past, guidance and navigation have involved intense ground-operation activity. However, on-board computers have become computationally powerful, lightweight, and energy efficient. Satellites now carry advanced on-board computers and are capable of performing autonomous navigation. Another important factor enabling a move to autonomous navigation is the development of accurate on-board sensors, such as Navstar. The principal problem remaining is that of providing the on-board computers with ephemeris data from a source that is reliable, robust, and economical in terms of both cost and weight.
Many autonomous navigation methods currently exist. For example, the Microcosm Autonomous Navigation System uses observations of the Earth, Sun, and Moon, and determines orbit, attitude, ground look point, and Sun direction. Its typically accuracy is approximately 100 m-400 m in a LEO system. Another navigational aid is the Space Sextant, which uses the angle between particular stars and the Moon's limb. The space sextant determines both orbit and attitude, and its typical accuracy is 250 m. Stellar refraction is another navigation system and uses the refraction of starlight passing through the atmosphere to determine both orbit and attitude. Its typical accuracy is 150 m-100 m. Yet another system is Landmark Tracking, which makes use of angular measurements of landmarks to determine both orbit and attitude. Its typical accuracy is measured in kilometers.
Possibly the most popular navigational system is Navstar, also known as the Global Positioning System (hereinafter "GPS"), which uses a network of navigation satellites. GPS is currently operational and primarily used for the determination of orbital information. However, attitude determination using GPS and multiple GPS antennas has been proposed. The positional accuracy obtainable from GPS is in the 15 m to 150 m range depending on whether the system is using military or commercial grade data. GPS receivers receive signals from four GPS satellites and use the received information to solve simultaneously for the three components of the observer's position and the current time. This calculation can be made several times, providing position and velocity information, which is in turn used to determine orbital parameters. The GPS constellation is at approximately half-geosynchronous altitude and works best for LEO satellites. Since GPS is operationally proven and at least as accurate as other known navigational systems, it is commonly used.
However, two significant problems with GPS are reliability and cost. The potential lack of availability of the GPS satellites for even a short period, due to either geometrical circumstances, the failure of one of more of the GPS satellites, or the failure of the on-board GPS receiver is a major concern for an expensive spacecraft which depends on GPS for attitude and positional determination. In addition, space grade GPS receivers can be prohibitively expensive, particularly when it is noted that critical satellite systems in need of accurate positional data may employ redundant GPS receivers on-board each satellite. The additional expense of a redundant GPS receiver, particularly to each satellite in a constellation potentially comprising dozens of satellites with limited lifespans, is substantial. Furthermore, there is a need for a method and apparatus which would effectively serve as a backup to an expensive primary navigational system such as GPS.
A need has long existed for an improved satellite ephemeris determination system.